I have Pietta army 1860, and have tested lee conical 450-200.
There is a slight "glitch" when loading, as in start the conical goes down the chamber under some angle, but ones they reach the first lube ring, they get upright as they are supposed to.
After loading the cylinder I saw no difference looking from top in the chamber allignement in all 5 conical bullets (1 chamber I ussually keep empty), all alligned the same way.
Afterwards, for shooting - no problem noticed.
The only drawback I noticed is less accuracy for conicals when compared to round ball (I use rb 454).
I atribute this problem to barrel rifling which in my Pietta army is designed for RB (not conical), which is around 1/28. For conical sharper rifling twist should be considered - i.e. Uberty, unless Pietta has another production line for which I am not aware.